CA1333282C

CA1333282C - Imbalance compensated drill bit

Info

Publication number: CA1333282C
Application number: CA000606272A
Authority: CA
Inventors: J. Ford Brett; Tommy M. Warren
Original assignee: BP Corp North America Inc
Current assignee: BP Corp North America Inc
Priority date: 1989-02-21
Filing date: 1989-07-20
Publication date: 1994-11-29
Anticipated expiration: 2011-11-29
Also published as: DE69007310T2; CN1045147A; DE69007310D1; RU2092671C1; US5131478A; EG19015A; EP0384734B1; NO900814D0; NO179592B; EP0384734A1; NO179592C; NO900814L; CN1042252C

Abstract

An imbalance compensated drill bit is disclosed that takes advantage of undesired and destructive imbal-ance forces to prevent bit whirl. Methods of designing and making such imbalance compensated drill bits are dis-closed whereby a drill bit body has at least one cutting zone with a plurality of cutting elements extending there-from and at least one bearing zone. The bearing zone has a relatively smooth surface and is located at a position where the net imbalance force (from the cutting elements) is directed towards. When the drill bit is rotated, the imbalance force presses the bearing zone against the bore-hole wall, and the bearing zone slips along the wall, thereby preventing the center of rotation to shift and create the destructive whirling motion.

Description

13332~2 IMBALANCE COMPENSATED DRILL BIT
BACKGROUND OF THE INVENTION
10 1. Field of the Invention The present invention relates to drill bits used to create boreholes through a material and, more partic-ularly, to such drill bits that are used in the explora-tion and production of hydrocarbons. Setting of the 15 Invention In the exploration and production of hydrocar-bons, a rotating drill bit is used to create a borehole through the earth's subsurface formations. The users of the drill bits and the drill bit manufacturers have found 20 that by controlling more precisely the weight-on-bit (WOB) and increasing the rotational speed (RPM) that increased penetration rates can be achieved. However, as the RPM is increased, the drill bit effective life has decreased dra-matically because the cutting elements on the drill bit 25 become cracked and occasionally are violently torn from the bit body.
Numerous studies have been made to find out what causes such destruction to the cutting elements. The inventors hereof have previously found that a substantial :13~39l~2 portion of the destructive forces are generated by radial imbalance forces that cause the drill bit to rotate about a center offset from the geometric center of the bit body in such a way that the drill bit tends to backwards whirl 5 about the borehole. This whirling causes the center of rotation to change dynamically as the drill bit rotates about the borehole. Thus, the cutters travel faster, side-ways and backwards and are subjected to greatly increased impact loads, thereby destroying the cutters.
More specifically, circumferential drilling imbalance forces exist to some degree on every drill bit and these forces tend to push the drill bit towards the side of the borehole. If the drill bit has a normal cut-ting structure, the gauge cutters are designed to cut the 15 edge of the borehole. During the cutting process, the effective friction between the cutters near the gauge area increase and, thus, the instantaneous center of rotation becomes some point other than the geometric center of the drill bit. When this happens, the usual result is for the 20 drill bit to begin to backwards whirl around the borehole.
The whirling process regenerates itself because sufficient friction is always generated between the drill bit gauge area and the borehole wall - no matter what the orien-tation of the drill bit - from the centrifugal forces gen-25 erated by the rapid acceleration of the drill bit.
Various methods and equipment have been proposedto eliminate or reduce these imbalance forces, including using dynamically balanced lower drillstring assemblies and realigning the cutters to reduce the imbalance forces.

~333~82 SUMMARY OF THE INVENTION
The present invention has been contemplated to overcome the foregoing deficiencies and meet the needs described above. By way of the present invention, the 5 imbalance forces existing on a drill bit are detected and measured. Modifications are made to the drill bit to not necessarily reduce the imbalance forces but to take advan-tage of these forces. Specifically, an existing drill bit or a yet to be completed drill bit have the imbalance 10 forces measured using a spatial coordinate system. The unbalance forces are resolved to generate a single force direction, a point or area on the drill bit body which will always be moved towards the borehole wall. No cut-ters are placed in this area so that a relatively smooth 15 bearing zone is defined. When the drill bit so modified is rotated, the unbalance forces push the bearing zone against the borehole wall but no whirling is generated because no cutters are on the bearing zone to dig into the borehole wall to create the whirling forces, i.e., the 0 bearing zone always slides along the borehole wall.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a perspective view of a drill bit embodying the present invention.

FIGURES 2A, B, and C show a perspective view of a portion of a bearing zone on a drill bit with different embodiments of wear surfaces thereon.
FIGURES 3A and B show a bottom view of a drill bit and the resulting forces at time t (Fig. 3A) and time t+~ (Figure 3B).

133~2 FIGURE 4 is a perspective view of a coordinate measuring machine and a drill bit.
FIGURE 5 is a view of an unworn PCD cutting sur-face.
FIGURE 6 is a view similar to Figure 5 showing a worn PCD cutting surface.
FIGURE 7 is a plan view of the drill bit with a selected cutter not yet installed.
FIGURE 8 is a flow chart illustrating the pre-10 ferred mode of implementing the method of the instant invention.
FIGURE 9 is a schematic diagram of a PCD cutting surface showing the side rake angle.
FIGURE 10 is a schematic diagram of a PCD cut-15 ting surface showing the back rake angle.
FIGURE 11 is a plot of the drill bit cutter sur-faces of a drill bit.
FIGURE 12 is a side view of a cutter embedded in a rock formation.
FIGURE 13 is a view of a PCD cutting surface taken along lines 13-13 in Figure 12.
FIGURE 14 is a diagram of a drill bit cutting surface embedded in a rock formation.
FIGURE 15 is a bottom view of a drill bit show-25 ing cutters to be removed to define a bearing zone.
FIGURE 16 is a graphical representation versus time of a torque and vibration output for a drill bit without a bearing zone drilling through Carthage material.

1333~.?
FIGURE 17 is a graphical representation versus time of a torque and vibration output for the drill bit of Figure 16 drilling through dolomite material.
FIGURE 18 is a graphical representation versus 5 time of vibrations and torque for the drill bit of Figure 16 drilling through Carthage material at increased WOB.
FIGURE 19 shows a side view comparison of bot-tomhole pattern predicted and as actually measured for a drill bit of Figure 16, but with certain cutters removed 10 to define a bearing zone.
FIGURE 20 is a graphical representation versus time of a torque and vibration output of the drill bit of Figure 16 drilling through Carthage material at elevated RPM.
Figure 21 is a graphical representation versus time of a torque and vibration output of the drill bit of Figure 16, with cutters removed to define a bearing zone, drilling under the same conditions as that for Figure 20.
Figures 22, 23, and 24 are graphical results of 20 tests of the drill bit of the present invention and an unmodified drill bit to show improvements in rate of pene-tration (ROP).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is an imbalance compen-25 sated drill bit and related methods of making such a drillbit from beginning or from an existing drill bit. The drill bit includes a bit body interconnectable to a source of rotation that has at least one defined bearing zone on a side portion thereof and at least one defined cutting ~33~
zone. A plurality of cutting elements extend from the at least one cutting zone and are arranged about a predeter-mined center of rotation spaced from the geometric center of the bit body. The cutting elements cause the at least 5 one bearing zone to be pushed towards the borehole wall.
Since the at least one bearing zone is relatively smooth, it slides along the borehole wall and does not dig into the subterranean material to create destructive whirling motion.
The methods of designing and thereafter building the drill bit of the present invention can be briefly sum-marized as follows. An array of spatial coordinates rep-resentative of selected surface points on the drill bit body and on cutters mounted thereon is created. The array 15 is used to calculate the position of each cutting surface relative to the longitudinal axis of the bit body and a vertical reference plane which contains the longitudinal axis of the bit body is established. Coordinates defining each cutter surface are rotated about the longitudinal 20 axis of the bit body and projected onto the reference plan thereby defining a projected cutting surface profile. In manufacturing the drill bit, a preselected number of or all of the cutters are mounted on the bit body. A model of the geometry of the bit body is generated as above 25 described. Thereafter, the imbalance force which would occur in the bit body (under defined drilling parameters) is calculated. The imbalance force and model are used to calculate the position of an additional cutter or cutters which when mounted on the bit in the calculated position would cause the imbalance force to be directed towards the at least one defined bearing zone on the bit body. Cut-ters are then mounted in the position or positions so cal-culated. Also, the drill bit can have an imbalance force 5 designed from inception so that when the cutters are placed on the bit body the drill bit has the desired imbalance force. In the case of modifying an existing drill bit, a cutter or cutters are removed from positions so calculated to define at least one bearing zone which 10 has the imbalance forces directed theretowards.
The following discussion will be divided into three parts: the drill bit itself, the methods of making the drill bit, and drilling test results. Drill Bit Fea-tures As shown in Figure 1, a drill bit 10 includes a generally cylindrical body 12 and can be Stratapac, PDC, diamond matrix, roller cone or other similar type bit design and configuration. In the embodiment shown in Figure 1, the body 12 includes a threaded pin shank 14 for interconnection with a source of rotation, such as a down-hole motor, or rotating drillstring, as is well known. In one embodiment, a plurality of cutting blade members 16 extend from the body 12 and include a plurality of cutting elements 18 mounted thereon in any conventional manner.
The blades 16 and cutters 18 define at least one cutting zone of the bit body 12. At least one relatively smooth, hardened pad area 20 is provided on, and can extend from, the bit body 12 towards which the net imbalance forces are directed. The pad area 20 can, as shown in Figures 2A, 2B

~3~28,~
and 2C, include a wear coating 22, a plurality of diamond stud inserts 24 or thin diamond pads 26. Further, the pad area 20 is preferably of sufficient surface area so that as the pad area 20 is forced against the borehole wall, 5 the applied force per square inch will be much less than the compressive strength of the subsurface material. This preferable requirement is to keep the pad area 20 from digging into and crushing the borehole wall, which would result in the creation of the undesired whirling motion.
The pad area 20 defining a bearing zone can be two or more pad areas equally spaced about the center line of the side forces. One such embodiment would comprise two relatively smooth sections protruding slightly from the bit body 12 on either side and equally spaced from the 15 side force center line. The pad area 20 defining a bear-ing zone can comprise cutters of a different size, config-uration, depth of cut, and/or rake angle than the cutters in the cutting zone. These different cutters would gener-ate less cutting forces than the cutters in the cutting 20 zone so the different cutters could still be considered relatively smooth as compared to the cutting zone.
Another alternate embodiment would include one or more cylindrical rollers or caged ball bearings extending a rolling surface out from the bit body 12 to permit the 25 bearing zone to more easily roll across the borehole wall.
The bearing zone can extend over as long or as small an area of the bit body as desired, with the con-straints being having sufficient cutters and cutter arrangement in the cutting zone for efficient cutting of :1~33~

material. Further, the bearing zone can extend across the side portion of the bit body and downwardly onto a rounded face portion of the bit body if desired.
The bit body 12 can include a smaller diameter 5 cutting zone, usually referred to as "pilot section", that extends coaxially out from a larger diameter cutting zone.
One or more bearing zones or pad areas 20 can be located on the smaller, larger or both cutting zones. Preferably, the pad area 20 would be adjacent the smaller diameter 10 cutting zone because the pad area 20 will be close to the center of the bit body 12 as possible which can reduce the torque that tends to cause the drill bit to roll onto the cutting zone and thus whirl.
In operation, the pad area 20 prevents the whir-15 ling motion from starting in the following manner. As shown in Figure 3A, a drill bit body 12 is shown rotating within a borehole 28. The plurality of cutters 18 spaced about the drill bit body 12 create an imbalance force that is directed towards one area of the bit where cutters have 20 been removed from and/or replaced with a relatively smooth bearing zone. As shown in Figure 3B, the bit body 12 has been rotated to a new position at a time t+~. Because of the pad area 20, there is a greatly reduced number or no means for the bit body 12 to dig into the borehole wall 28 25 to create the whirling motion, because the pad area 20 slips or slides along the borehole wall without creating sufficient force to affect the center of rotation of the drill bit. Note the resulting force arrows originate at the same point, i.e., the center of rotation, not the _g_ ~ 3~ ~,J

geometric center of the drill bit. Thus, the center of rotation of the drill bit is between the center of the borehole and the bearing zone.
The use of the pad area 20 has the advantage of 5 being relatively insensitive to bit wear, formation inho-mogeneities and operating conditions. The imbalance force generated by the cutters change with wear, formation and operating conditions; however, the direction of the imbal-ance force does not significantly vary. Thus, this type 10 of arrangement can be made to work to prevent bit whirl on most all configurations of drill bits.
The drill bit can be designed to have a greater mass on its side adjacent the pad 20 so that centrifugal forces push on the pad area 20, even if the drill bit 15 rotates about its center axis. Another advantage is that if the drill bit does drill an overgauge hole (a hole larger in diameter than the outside diameter of the drill bit itself), the centrifugal force generated by the rotat-ing imbalanced mass will act as a stabilizing force. Any 20 perturbations, such as formation inhomogeneities, that may tend to counteract the imbalance force holding the pad area 20 on the borehole wall will be resisted by the cen-trifugal force generated during the normal drilling opera-tion.
METHOD OF MAKING THE DRILL BIT
The method of this invention uses a geometric model of the cutting surfaces on a drill bit to calculate the forces acting on each of the cutting surfaces. In one aspect of the invention, a model is used to build a drill 13332~;~
bit. In so doing, a major portion of the cutters are installed on the drill bit, a geometrical model of the bit is generated and the forces acting on each of the cutters for given drilling conditions are calculated. Thereafter, 5 exact positions for the remaining cutters to be installed are calculated with the calculated position minimizing the radial component of the forces acting on the cutters. The remaining cutters are then installed in the calculated positlon .
Turning now to Figure 4, indicated generally at 64 is a commercially-available coordinate measuring machine shown adjacent bit 40. The coordinate measuring machine includes a pointer 66 fixedly mounted on a slida-ble arm 68. The lower end of pointer 66 is formed into a 15 point which is fixed relative to arm 68.
Arm 68 is mounted on a laterally slidable frame 70. Frame 70 includes parallel rods 71, 73 along the axis of which frame 70 may slide. A meter 72 indicates the lateral position of frame 70 relative to an upright base 20 74.
Frame 70 is also vertically moveable along par-allel rods 76, 78 with the height of the frame being indi-cated by a meter 80.
Parallel rods 82, 84 are mounted on a lower 25 fixed base portion 86. Rods 82, 84 support upright base 74 for sliding movement along the axis of rods 82, 84. A
meter (not visible) indicates the relative position of base 74 on rods 82, 84. Rods 82, 84 are oriented in space perpendicular to rods 76, 78 and to rods 71, 73. Like-wise, rods 76, 78 and rods 71, 73 are each perpendicularto each of the other two sets of rods.
The readings on the meters indicate the relative positions of the rods used to define a point in space 5 occupied by the pointed end of pointer 66. The position of the point on the pointer can thus be referenced to a three-dimensional coordinate system defined by X, Y and Z
axes with each meter representing a relative position along one of the axes. A digital meter 88 provides a 10 read-out of the X, Y and Z coordinates of the point on pointer 66 and also provides such coordinates, upon opera-tor command, to the memory of a commercially available computer (not shown).
Drill bit 40 is mounted on a rotary turntable 15 90, the angular position of which is controlled by handle 92. An angular scale, not visible, shows the angular position of the turntable and thus of drill bit 40 which is supported thereon with its axis aligned with the turnt-able axis.
In the instant mode of implementing the method of the invention, pointer 66 is positioned on a plurality of points on the surface of the drill bit and the coordi-nates of each particular point are stored in the computer.
From this data, a computer model of the drill bit is con-25 structed. In making the measurements, a first set of mea-surements is made around the side of the bit so that the computer has data from which the longitudinal axis of the bit can be determined. A second set of measurements on the perimeter of each cutter face is made. In making the ~ ~3~2~
measurements, the angular position of rotary table 90 isnoted and is associated with the three values which are produced by measuring machine 64 for all measurements taken at that particular angle of the rotary table. This 5 enables all measurements to be taken substantially normal to each measurement point and increases the accuracy of the measurement process.
After the bit is rotated 360 and several points are measured about the circumference thereof and recorded, 10 each cutter face on the cutters is measured.
For a description of the manner in which these measurements are made attention is directed to Figures 5 and 6. Each cutter face includes a vertical axis 94 which is substantially parallel to the cutter face and extends 15 from the uppermost portion thereof to the lowermost por-tion. Also included is a horizontal axis 96 which extends from the leftmost to the rightmost portions of the cutter face and is parallel thereto. In making the measurements with the coordinate measuring machine, the point on poin-20 ter 66 in Figure 4 is first positioned at the intersectionof axis 94 with the perimeter of cutter face 62, such defining a first measurement point 98. A second measure-ment point 100 is located at the intersection of axis 94 with the lower edge of cutter face 62. A third measure-25 ment point 102 is at the left side intersection of axis 96with the outer perimeter of cutting face 62 while a fourth measurement point 104 is at the right side intersection of axis 96 with the perimeter of cutting surface 62.

The numbers and arrows shown in the central por-tion of cutting face 62 in Figures 5 and 6 indicate the order in which the first four measurements on each cutting face on the drill bit are taken: along the cutting face 5 vertical axis first and thereafter along the cutting face horizontal axis. When the point on pointer 66 is posi-tioned first at point 98, the coordinates and angular position of the turntable are provided to the computer and likewise for each of the other four measuring points.
Figure 6 i$ a view of cutting surface 62 after the bit has been used to drill a bore and thus includes a wear flat 105 on one side thereof developed as a result of the cutter being urged against the rock formation during drilling. When such irregularities occur on the perimeter 15 of the cutting surface as in the case of Figure 6, fifth and sixth measurement points 106 and 108 are taken in order to completely define the perimeter of the cutting face.
As each measurement is put into the computer, it 20 is associated with a number which indicates the order in which the measurement was taken. In Figure 5, the meas-urements at points 98, 100, 102 and 104 are numbered 1, 2, 3, 4, respectively, and in Figure 6, the measurements are similarly numbered with measurements at points 106 and 108 25 being additionally numbered 5 and 6, respectively. Each cutting face is measured at a single angle on the turnta-ble which is also recorded. In addition to the foregoing, a value is recorded to indicate the general shape of the edge of the cutting face between adjacent measurements.

1~3~8~
If the shape is generally a straight line, a zero isrecorded and if the shape is generally a circular arc, a one is recorded. Thus, a number is provided to the com-puter memory to indicate the general shapes between each 5 of the adjacent measuring points in Figure 5.
In Figure 6, a number value of one is recorded between the first and fourth measurements, between the fourth and second measurements, between the second and third measurements, between the third and fifth measure-10 ments, and between the sixth and first measurements whilea zero is recorded between the fifth and sixth measure-ments to indicate the substantially straight line edge formed by worn portion 105. Thus, each of the recorded measurement points defines the perimeter of a cutting sur-15 face having a fixed angular orientation relative to thelongitudinal axis of the drill bit. In addition, the con-nectivity between each adjacent point is stored in the computer memory. The connectivity is simply an indication of the shape of the cutting face perimeter between adja-20 cent measurements. As will later become more fully appar-ent, the connectivity value between adjacent measurements is used to interpolate additional coordinates using circu-lar interpolation, when the connectivity is one, and linear interpolation, when the connectivity is zero.
Turning now to Figure 7, drill bit 40 is shown partway through the process of manufacture. As can be seen, cutters are all mounted on drill bit body 41, except for cutter 8. A bore 114 is formed in body 41 to receive the stud of cutter 8. Each of the other cutters has its 1333~2 stud press fitted into an associated bore in the drill bit body. Prior to mounting cutter 8 on the drill bit body, the dimensions of the drill bit body around the circumfer-ence thereof and the cutting faces of each of the cutters 5 which are installed on the drill bit body are recorded and entered into a computer memory as previously described.
Thus, the computer has data relating to the circumference of the drill bit body (from which the bit axis designated by axis 42, can be determined) and the position of each 10 cutter face (except, of course for cutter 8 which is not yet installed) in space relative to the bit axis.
Turning now to Figure 8, included therein is a flow chart of a computer program for use in connection with manufacturing a drill bit. Although the entire flow 15 chart deals with the manufacture of the drill bit, a sig-nificant portion of the computer program relates only to generating a model of a drill bit. As will later become more fully apparent, that portion of the program relating to modeling the bit begins at box 110 with the step of 20 "Locate Bit Center" and concludes with box 112, "Write:
Measured and Interpolated Coordinates in Two-Dimensional Array."
To initiate the program, data is provided relat-ing to the strength of the rock in which the bit is to be 25 used, the rate of revolution of the bit and the rate of penetration, i.e., the rate at which the hole is bored.
Also, the bit body coordinates (those taken about the cir-cumference of the drill bit body) are read from the com-puter memory.

:~33328~
Thereafter, the bit body coordinates are used to locate axis 42 by means of a least squares regression. A
subroutine to accomplish this task can be written by a person having ordinary skill in the art.
As will be recalled, each of the three coordi-nates for each point measured on the bit body are refer-enced to the coordinate measuring machine rather than to the longitudinal axis of the drill bit body. After the longitudinal axis of the drill bit body is located in the 10 coordinate system in which the measurements were taken, the coordinate system can be translated to set the verti-cal or Z axis to align with the bit center. Next, the data file for a particular cutter number and the rotary angle at which that data was generated is read from the 15 computer memory. Thereafter, each measurement number, for example, one of a series of sequential numbers identifying the order in which the measurements were taken, is read with the coordinates associated with that particular meas-urement number. Then, the connectivity between adjacent 20 measurements is read which, as will be recalled, defines the general shape, either straight line or generally cir-cular arc, between adjacent measurements.
Next, the side rake of each cutter face is calculated. The side rake is defined relative to a verti-25 cal reference plane which contains axes 44, 46. The planepasses through the center of the drill bit body and divides it into equal halves. Coordinates which define the horizontal cutting face axis for a particular cutter, such being measurement points 102, 104 in Figure 5, are I 3~28~
rotated about the circumference of the drill bit centeralong the path the coordinates would travel during actual drill bit rotation. When the center point of the cutter face intersects the vertical reference plane, the angle 5 between axis 96, the horizontal axis, and the vertical reference plane defines the side rake. It can be seen that the coordinates located at the center point of each cutter surface can be easily calculated since the same is defined by the intersection of axes 94, 96, the position 10 of which are known.
In a similar fashion, back rake is defined as the angle between reference plane 116 and vertical axis 94 after the coordinates defining the horizontal and vertical axes are rotated until the intersection thereof intersects 15 the reference plane. In other words, to calculate both side rake and back rake, the coordinates defining the cutter face are first rotated until the intersection of axes 94, 96 is received in the vertical reference plane.
Thereafter, the angles between horizontal axis 96 and the 20 reference plane (side rake) and vertical axis 94 and the reference plane (back rake) are measured. It is to be appreciated that a subroutine capable of rotating the coordinates and measuring angles as above described could be easily written by a person having ordinary skill in the 25 art.
By way of example, Figure 9 is a top view of the drill bit body showing a vertical reference plane 116 which contains axes 42, 46. A cutter surface 62, repre-sentative of one of the cutter surfaces on drill bit 40, ~ 333282 has been rotated until the center thereof intersects plane116 as shown. It can be seen that since surface 62 is parallel to the longitudinal axis 42 of the drill bit body, there is zero degrees back rake. Thus, the angle 5 shown in Figure 9 is the side rake.
Figure 10 is a view of a cutter surface from the side of the drill bit. Cutter surface 62 has been rotated until the center of the same intersects plane 116. Sur-face 62 in Figure 10 has zero degrees side rake since the 10 surface is parallel with axis 42 and the angle shown in back rake.
It is to be appreciated that in most cases, cutter surfaces include both slight amounts of back rake and side rake. The views of Figures 9 and 10 are for the 15 purposes of illustrating the manner in which back rake and side rake are measured.
Turning again to the flow chart of Figure 8, after calculation of side and back rakes for a particular cutter surface, the program selects a measurement point on 20 the circumference of the cutter surface and checks the connectivity between that point and the next clockwise measurement point. If the connectivity is zero, a linear interpolation is run between the adjacent coordinates to establish a series of coordinates along a straight line 25 between the adjacent measured points. The program contin-ues to the next clockwise measuring point, checks the con-nectivity between the adjacent points and if equal to 1, generates a series of coordinates by circular arc interpo-lation between the adjacent points. The program continues 133328~
in a clockwise fashion about the cutter surface until a plurality of coordinates are produced by interpolation between adjacent measuring points which define the perime-ter of the cutter surface. A loop 118 continues until 5 coordinates have been interpolated between all measuring points thus defining the cutter face perimeter. Next, the program projects both the measured and interpolated coor-dinates into reference plane 116. Thus, each coordinate in the projected cutter face profile can be designated by 10 two numerals and the measured and interpolated coordinates which define the perimeter of the cutter face are stored in a two-dimensional array. By way of example, attention is directed to Figure 11 which is a plot of each of the cutter faces on drill bit 40 in Figure 7 projected into 15 reference plane 116. With the vertical axis corresponding to the drill bit body axis 42, each coordinate in the per-imeter of a cutter face profile can be designated by a distance along a radial axis and a distance above the radial axis. For example, on the horizontal axis, zero is 20 at the center of the bit body and 4.25 inches is at the circumference of the bit body since drill bit 40 in this example is an 8.5 inch bit.
The profile in Figure 11 includes an upper pro-file, designated "all cutters" which is how each of the 25 cutter surfaces appears projected onto the reference plane. In order to more clearly identify the cutter sur-faces in each of the three spirals, a projection onto the reference plane for each spiral is shown beneath the proection for all cutters. It can be seen in spiral ~ 33~2~2 number 1 that there is no profile for cutter 8 since thesame has not yet been installed.
Returning attention again to the Flow Chart of Figure 8, after each of the cutter faces in cutters 1-7 5 and 9-36 have been represented in a two-dimensional array as described above, the program proceeds to box 119 and the step of calculating the forces acting on each cutter is undertaken. Considering now Figures 12 and 13, gener-ally speaking the forces acting on an individual one of 10 the cutters on the drill bit can be defined as a normal or penetrating force, such being designated Fn in Figures 12 and 13, and a cutting force such being Fc in Figure 12.
The normal force is the force required to cause the cutter to penetrate into the rock and is given by the formula:

F = lCsois (( - EBR) d B RS d C + A RS C

In the above formula, alpha is the angle of the 20 cutter from the X axis, in Figure 7, which serves as an arbitrary reference axis which is parallel to axis 46 and, like axis 46, is contained in plane 116. EBR is the effective back rake which is a function of the real back rake and real side rake, both of which were discussed pre-25 viously, and the angle at which Fn acts.
Referring to Figure 14, cutting surface 62 isschematically illustrated embedded in a rock formation 120. Although not shown in Figure 14, most of the other cutting surfaces on the drill bit body are embedded to one 1~332~

extent or another in formation 120. The effective backrake (EBR) may be thought of as the angle between cutting face 62 of the cutter and a cutting plane 122. Cutting plane 122 is parallel to an axis formed between points 124 5 and 126 on the cutting face and is perpendicular to Fn.
Points 124 and 126 are the points at which the surface of formation 120 intersects cutting face 62. In other words, the shaded area on face 62 defines the cross-section of the cut in formation 120 being made by cutting face 62.
Plane 122 is further oriented in that a perpen-dicular axis 128 to plane 122 passes through the longi-tudinal axis of the bore being drilled. Of course, when there is no wobble of the drill bit during drilling, the longitudinal axis of the bore and the drill bit are coin-15 cident.
In summary, the effective back rake is the angle between cutting face 62 and cutting plane 122 as shown in Figure 14. The effective back rake can be computed when the real side and back rakes are known, which will be 20 recalled, were calculated by the program and when the position of cutting plane 122 is known. The position of cutting plane 122 is dependent upon the depth of the cut which, in turn, is dependent upon thè rate of penetration and bit revolution. As will be recalled, these values are 25 input to the program as preselected parameters indicative of the drilling conditions under which the bit will be used.
Bf is the bit factor, a variable which ranges between about 0.75 and about 1.22 and which in the instant mode of implementing the invention is selected to accountfor slight differences between simulations on the computer model of bit wear for a given drill bit and real drill bit wear occurring under actual conditions. The bit factor, 5 Bf, accounts for unexplained effects for a particular bit.
A value of this factor greater than 1.0 indicates that the bit drills slower than expected and a value less than 1.0 indicates that the bit drills faster than expected. A
person having ordinary skill in the art can empirically 10 determine the value of Bf for a selected drill bit.
The width of cut made by the cutter is desig-nated dw. In the instant mode of implementing the invention, the computer model generates a grid of parallel vertical lines across each cutter face and dw is equal to 15 the width between adjacent lines. The equation is then calculated for each grid to generate a total force for the cutter.
The effective depth of cut is designated dCe and C1 is a dimensionless constant, which in the instant 20 manner of implementing the invention is equal to 1,100.
The manner in which C1 is determined will be shortly described, and dCe can vary slightly from the actual depth of cut. The manner of selecting the value of dCe is known to persons having ordinary skill in the art.
Considering now the second term of the equation for F, Aw is the wear flat area, RS is again a constant related to the strength of the rock formation and C2 is a constant equal to 2,150.

1333`282 The first component in the equation for Fn is equal to the amount of downward force required to prevent the cutter face from riding up out of the cut for a selected cutting depth and width. The second component of 5 the equation for Fn includes a factor for a dull cutter which has a wear flat of area Aw formed thereon. This portion of the normal force is that required to compress the rock beneath a dull cutter to enable it to penetrate the rock. Cl and C2 can be empirically determined by 10 first using a new bit thus setting Aw to zero and there-fore causing the entire second term to go to zero. A
known normal force can be applied and with each of the other factors known, Cl can be determined. Thereafter, the value of Cl, 1,100 in the instant mode of implementing 15 the invention, is inserted into the equation and the bit is used until wear flats appear. Thereafter, wear flat area is measured and inserted into the equation which is then solved for C2, which in the instant mode of imple-menting the invention equals 2150.
The circumferential cutter force, Fc in Figure 12, is that required to advance the cutter along the cut after the normal force embeds the same in the for- forma-tion. An arrow depicting the orientation of Fc is also shown in Figure 7. The circumferential cutter force is 25 dependent upon the sliding friction between the cutter and rock and the force required to fracture the rock. The following equation can be used to calculate the circumfer-ential cutter force:

I3332g2 c l-Sin (a - BR) c3 RS dw C4 FN

The first term of the circumferential cutter force equation is the cutting force, i.e., that required to fracture the rock, and the second term is the nonpro-ductive friction carried on a cutter wear flat. The vari-ables in the equation are as described above and, in addition, dCm is the mean depth of cut. In the instant mode of implementing the invention, the dimensionless con-stant C3 and C4 are equal to 3,000 and 0.3, respectively.
C3 and C4 can be determined empirically by drilling with two known circumferential forces applied to the drill bit, inserting all the known variables into the circumferential cutting force equation at each value of circumferential force and solving both equations for C3 and C4.
In the example under consideration, i.e., drill bit 40 in Figure 7, the value of the circumferential and normal forces, as illustrated in Figure 12, at each cutter is calculated. As noted above, the depth of cut is a function of the rate of penetration and the bit rotation rate which are both provided to the computer as prese-lected values. Since the cutters on the drill bit can cut on a surface that is inclined to the vertical by an angle beta, illustrated in Figure 14, the normal force can be resolved into a vertical and radial component and the cir-cumferential force can be resolved into radial components, and a moment about the bit center. The radial component 13332~2 of the normal force, identified as Fr Figure 14, is equal to Fn sin(~).
The components of the normal force and the cir-cumferential force which act on the bit in the plane 5 normal to the bit rotational axis can be resolved into a single force acting on the bit center and a single couple, both lying in the normal plane. The couple is the torque required to rotate the bit and the force is the imbalance force, i.e., that force which tends to push the bit 10 against the side of the bore.
It is helpful in computing the magnitude and direction of the imbalance force to resolve the cutter forces into components along the X and Y directions as referenced in Figure 7. As mentioned, these axes are 15 arbitrarily chosen but are fixed relative to any partic-ular identifying feature on the bit. The vertical pene-trating force, Fv has no component in these directions.
The radial penetrating force (Fr) of the normal force (Fn) can be resolved into components along the X and Y axes by 20 the following equations:

Fx_r = Fr * cos(alpha) Fy_r = Fr * sin(alpha) Since the circumferential force acts at right angles to the radial force for each cutter, it can be 133~282 resolved into components in the X and Y directions with the following equations:

x-c Fc * cos(alpha - 90) Fy-c = Fc * sig(alpha - 90) It is to be appreciated that at each cutter, 10 there is no radial component of Fc however, when the value of Fc at each cutter is resolved into components along the X and Y axes in Figure 7 with those vectors being summed, there can be a total radial component of the circumferential force. The total X and Y
15 components of the imbalance force is then obtained by sum-ming the components from the individual cutters as fol-lows:

FXt = Fx_r + Fx-c Fyt = Fx-r + Fy-c After such summing, the magnitude of the radial imbalance force is given by:

Fi lFxt + Fyt Returning again to the flow chart of Figure 8, it can be seen that the step identified in box 130 is per-formed by resolving the cutter forces in a plane perpen-dicular to the drill bit axis into a single imbalance 5 force as described above. In a similar manner, a moment that tends to tilt the drill bit in a plane parallel to the central axis is calculated.
The final step in the flow chart is identified as "Calculate Position of Cutters." In the example under 10 consideration, there is only one cutter, cutter 8, remain-ing to be mounted on the drill bit body. An iterative process can be used to calculate the position for cutter 8 which directs the imbalance force towards the bearing zone. First, it can be seen that the cutter can be radi-15 ally positioned about the longitudinal axis of the cutterstud within bore 114, and further can be installed at depths which vary from completely seated, i.e., with the stud being received abutted against the lower end of bore 114, to some position thereabove. Initially, an arbitrary 20 back and side rake and vertical position of the cutting face, within preselected ranges, is assigned to cutter 8 and the program to model the drill bit and calculate cut-ting forces is rerun with cutter 8 in the assigned posi-tion. The program is repeatedly rerun with the face of 25 cutter 8 being repositioned in a direction which tends to increase and properly direct the imbalance force. The program ultimately produces a set of coordinates which identify a position for the cutting face of cutter 8.
Thereafter, cutter 8 is installed with care being taken to position the cutting face thereof in the calculated posi-tion.
The following Table I provides an output gener-ated after placement of cutter 8. Calculated values 5 include the volume of cut (volume removed in one revo-lution) and velocity of each cutter. The given rotary speed and penetration rate are shown below the table.
Wear flat area is calculated in a known manner for 5.0 hours drilling. Percent imbalance is the imbalance force 10 expressed as a percentage of weight-on-bit, which is the total Of Fyt for each cutter.
It should be appreciated that the method is not restricted to simply positioning a cutter within a pre-drilled bore. The program can be used to select the posi-15 tion for the bores in the drill bit body of one or morecutters after a preselected number of cutters are installed and the program run to determine the imbalance force. An iterative process similar to that described above can be used to position two or more remaining cut-20 ters either within predrilled bores, as in the exampleunder consideration, or the program can be used to deter-mine the position of bores to be drilled.
Moreover, the program is not necessarily limited to cutters of the type having studs extending therefrom 25 which are received in bores in a drill bit body. The same program can be used to position cutters which are directly affixed to a drill bit body by brazing or other known techniques.

It can be seen that program permits manufacture of the drill bit with the initial set of cutters being installed with relatively gross manufacturing tolerances as to the position of the cutter faces. Thereafter, when 5 the position of the balancing cutter or cutters is deter-mined, which properly directs the imbalance force, great care can be taken to so position the final cutters in order to obtain the desired result. Thus, the method of the invention enables installation of the great majority 10 of cutters under relatively gross tolerances thus saving time and money in the manufacturing process. In addition, the program generates a location for the final cutter or cutters which consistently properly directs the imbalance force in the drill bit so manufactured. Average quality 15 of bits so manufactured is therefore greatly increased over the average quality of bits manufactured in accord-ance with prior art methods.
Further, the present invention can easily be used to modify an existing drill bit to include the 20 defined bearing zone in a proper location. This modifica-tion can be accomplished by determining the direction of the imbalance force, as described above, and then elimi-nation cutters on an area where the imbalance force is directed towards. The removed cutters can be replaced 25 with a built-up pad, a plurality of studs or pads. A
number of such iterative steps may need to be taken to ensure that the quantity of imbalance force is acceptable and directed in the proper location. Such iterative steps can include replacement, removal and rearranging of cut-ters to achieve the desired results. Drilling TestResults A commercially-available PDC drill bit was selected because it has a very imbalanced design and 5 drills poorly. To ensure that this poor performance was not representative, three different drill bits of the same manufacturer, model and size were tested and all performed similarly. The primary cause of the poor performance is due to bit whirl, which causes the drill bit to drill a 10 lobed bottom hole pattern. In order to test the present invention, one of the selected PDC drill bits was modified to incorporate a low friction bearing zone. This resulted in a drill bit that was adequate to test the low friction gage concept, but was not an optimized design.
Figure 15 shows the drill bit's cutter layout and the calculated direction of the about 2100 lb imbal-ance force is indicated. The magnitude and direction of this imbalance force was calculated with the PDC drill bit model computer program, described fully above. To define 20 the bearing zone, cutters were removed as indicated on the side of the bit to provide a relatively smooth sliding surface, rather than a cutting surface. Even though the bearing zone is referred to as low friction, the partic-ular geometry of this drill bit still provided fairly high 25 friction.
It is first beneficial to review the performance of the PDC drill bit before the cutters were removed in order to evaluate the benefit of removing the cutters.
The bottom hole patterns showed whirling. Figures 16 and 17 show typical plots of the vibrations that occurred with the drill bit at 120 RPM and drilling through Carthage and Dolomite. Figure 18 even shows that the PDC drill bit whirled at 1800 lb WOB and 120 RPM.
After removing the selected cutters to define a bearing zone located where the imbalance forces were directed towards, the drill bit was rerun in Carthage and Dolomite at conditions identical to the previous tests before removing cutters. Bottom hole patterns for the 10 Carthage and Dolomite tests were extremely smooth. TheCarthage hole was completely gage and the Dolomite was only 1/16 in. over gauge. Figure 19 shows a comparison of the bottom hole pattern predicted by the computer program, described fully above, and that measured from the Carthage 15 test. The excellent agreement between the prediction and the actual indicated that the drill bit was loaded as pre-dicted by the model and should provide a very long wear life.
The same PDC drill bit was tested on a turbine 20 drive at high rotational speeds before the cutters were removed. The bottom hole pattern in Carthage showed very definite whirling patterns and Figure 20 shows the vibration data recorded during these tests. Figure 21 shows the much reduced vibration data for the drill bit 25 after removing the cutters. Further, the borehole was only 1/8 in. over gauge and there was absolutely no evi-dence of whirl patterns.
Rock flour was plowed up by rounded carbide wear buttons placed where the cutters were removed on the "low 13332~2 friction" surface. More flour accumulated during theDolomite test (softer rock) than during the Carthage test.
The fact that the rock flour could only be seen on the buttons on the area where the cutters were removed further 5 indicates that the proper location on the bit was selected for cutter removal to provide the low friction bearing zone.
Penetration rates obtained with the PDC drill bit before and after the cutters were removed is not as 10 easy to quantify because normal performance tests could not be run with the drill bit in its original condition because of high vibrations. Figure 22 shows a comparison of the drill bit in Dolomite at 120 RPM before and after the low friction, bearing zone was formed. The pene-15 tration rate is about the same for both conditions, butduring the first tests (with no removed cutters) 23 out of 42 cutters were chipped. These chipped cutters may have resulted in a reduced ROP on subsequent tests. Figure 23 shows two tests in Carthage that indicate the ROP may have 20 been slightly higher before the low friction bearing zone was formed, but the original tests were conducted before the cutters were chipped in Dolomite. By these tests, the low friction bearing zone may not have greatly improved the penetration rate, but it certainly did nothing to hurt 25 it.

Figure 24 shows a comparison of the penetration rate obtained at 1000 RPM with the drill bit after the low friction bearing zone was formed compared to the perform-ance at 60 RPM before it was formed. Initially the rpm 13332~
was limited to 60 RPM because of the high vibrations, butafter the bit was modified 1000 RPM could easily be run.
In fact the vibrations were lower at 1000 RPM than they were at 60 RPM before the drill bit was modified. The 5 combination of high ROP and low vibrations obtained with the crudely modified drill bit demonstrate the potential of the present concept as a means for providing a very acceptable high speed drill bit that exhibits no destruc-tive whirling.
Whereas the present invention has been described in relation to the drawings attached hereto, it should be understood that other and further modifications, apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.

Claims

1. A drill bit operable with a rotational drive source for drilling in subterranean rock materials to create a borehole, comprising:
a drill bit body having two ends, having a longitudinal bit axis between said ends, having means for connecting to a rotational drive source at one of said ends, having a cylindrical gauging portion at the exterior of said body andbetween said ends, and having an end operating portion at the other end of said drill bit body;
rock-cutting elements that are fixedly located on said drill bit body, that project from said drill bit body to cut the subterranean rock and form a borehole when the drill bit is rotated in the borehole against the rock, that are spaced apart from one another, and that produce, in response to rotating the drill bit in the borehole, a net radial imbalance force having a magnitude and having a direction that is substantially perpendicular to said longitudinal bit axis; and a bearing surface that is located on said gauging portion of said drill bit body at the intersection with a force plane defined by said longitudinal bit axis and the line of action of said net radial imbalance force and that is adapted toslidingly contact the wall of the borehole.

2. The drill bit of claim 1, wherein said bearing surface is substantially devoid of rock-cutting elements.

3. The drill bit of claim 2, wherein the bearing surface is a substantially smooth and wear-resistant sliding surface that is located in said cutter devoid region for slidably contacting the borehole wall during drilling.

4. The drill bit of claim 3, wherein said substantially smooth sliding surface contacts the borehole wall without hydrodynamic lubrication.

5. The drill bit of claim 3, wherein said substantially smooth sliding surface includes a wear resistant coating.

6. The drill bit of claim 3, wherein said substantially smooth sliding surface comprises a wear resistant diamond impregnated material.

7. The drill bit of claim 3, wherein said sliding surface has a first density;
and wherein adjacent portions of the drill bit body other than the sliding surface have a second density different from the first density.

8. The drill bit of claim 2, wherein said rock-cutting elements are non-symmetrically located on said end operating portion of said drill bit body.

9. The drill bit of claim 1, wherein said bearing surface has a plurality of wear elements.

10. The drill bit of claim 1, wherein said bearing surface is a substantially smooth hard surface.

11. The drill bit of claim 1, wherein said bearing surface has an area that is large enough that the force imposed on said borehole wall is less than the compressive strength of the material surrounding the borehole.

12. A method for making a drill bit of the type having a bearing zone on a side portion of a bit body and a cutting zone with a plurality of cutters mounted on the bit body, comprising the steps of:
mounting a preselected number of cutters within the cutting zone on the bit body;
generating a model of the geometry of the bit body and cutters mounted thereon;
calculating an imbalance force which would occur in said bit body under pre-defined drilling parameters;
using said imbalance force and said model to calculate the position of at least one additional cutter which, when mounted within the cutting zone on the bit body in said calculated position, would create a net imbalance force that is directed towards the bearing zone, that lies in a force plane defined bysaid longitudinal bit axis and the line of action of said net imbalance force and that has a magnitude to slidingly maintain the bearing zone in contact with the wall of the borehole; and mounting said at least one additional cutter within the cutting zone on said bit body in said calculated position.

13. The method of claim 12, wherein the step of generating a model of the geometry of the bit body and cutters mounted thereon comprises the step of determining the spatial coordinates of a plurality of points on said bit body and on each of the cutters mounted thereon.

14. The method of claim 13, further including the step of calculatlng the rake angle of the cutting surface of each cutter mounted on said bit body using said spatial coordinates.

15. The method of claim 14, further including the step of generating additional spatial coordinates for each cutting surface by interpolating between the spatial coordinates determined on said cutters.

16. The method of claim 12, wherein each of said cutters includes a cutting surface; wherein said parameters are bit rotation rate, penetration rate and rock strength; and wherein the step of calculating the imbalance force which would occur in said bit body for pre-defined drilling parameters comprises the steps of:
calculating the relative positions of said cutting surfaces on said cutters;
calculating the forces acting on each cutter for a pre-selected bit rotation rate, penetration rate and rock strength; and summing the radial forces acting on each cutter.

17. A method for making a drill bit of the type having a bearing zone on a side portion of a bit body and a cutting zone with a plurality of cutters mounted on the bit body, comprising the steps of:
mounting a preselected number of cutters, each of which defines a cutting surface, within the cutting zone on the bit body;
defining a three-dimensional coordinate system within which the bit body is contained;
determining the coordinates of a plurality of points on the bit body and on each of the cutters mounted thereon;
storing the coordinates so determined in a memory;
calculating the position of each cutting surface relative to the longitudinal axis of the bit body using said stored coordinates;
calculating the forces acting on each of the cutters mounted on the bit body based on the calculation of each cutting surface position for pre-defined drilling parameters;
resolving the radial cutter force components of said calculated forces into a single radial imbalance force;
using said single radial imbalance force to calculate the coordinates of at least one additional cutting surface which if present would create a net radial imbalance force that is directed towards the bearing zone, that lies in a force plane defined by said longitudinal bit axis and the line of action of saidnet radial imbalance force and that has a magnitude to substantially keep the bearing zone in contact with the wall of the borehole; and mounting said at least one additional cutter on the bit body in a position which places the cutting surface thereof at the coordinates calculated in the previous step.

18. A method for making a drill bit of the type having at least one bearing zone on a side portion of a bit body and a cutting zone with a plurality of cutters mounted on the drill bit body, said method comprising the steps of:
creating an array of spatial coordinates representative of selected surface points on a bit body having a plurality of cutters mounted thereon;
using said array to calculate the position of each cutting surface relative to the longitudinal axis of the bit body;
preselecting values for selected drilling parameters which are representative of the conditions under which the bit will be used;
using the cutting surface positions and said preselected values to calculate the forces acting on each of the cutters mounted on said bit body;
resolving the radial components of said calculated forces into a single radial imbalance force;
locating on the bit body an area where said single radial imbalance force is directed towards; and removing cutters from said area to define at least one bearing zone on the bit body that lies in a force plane defined by said longitudinal axis and the line of action of said single radial imbalance force and that has a magnitude tosubstantially slidingly keep the bearing zone in contact with the wall of the borehole

19. The method of claim 18, wherein the step of using the cutting surface positions and said preselected values to calculate the forces acting on each of the cutters mounted on said bit body comprises the steps of calculating penetrating forces and calculating cutting forces.

20. The method of claim 18, wherein the step of using said array to calculate the position of each cutting surface relative to the bit body includes the steps of calculating each cutter's cutting surface back rake and calculatingeach cutter's cutting surface side rake.

21. A method for making a drill bit of the type having a bearing zone on a side portion of the bit body and a cutting zone with a plurality of cutters mounted on the bit body, comprising the steps of:
determining positions for a pre-selected number of cutters to be mounted within the cutting zone on the bit body;
generating a model of the geometry of the bit body and the cutters;
calculating the imbalanced force which would occur in said bit body under defined drilling parameters;
using said imbalanced force and said model to calculate a change in the position of at least one of the cutters such that, when all of the cutters are mounted within the cutting zone on the bit body, said imbalanced force is directed towards the bearing zone and in a plane defined by said longitudinal bit axis and the line of action of said imbalanced force, and has a magnitude tosubstantially continuously keep the bearing zone in sliding contact with the wall of the borehole; and mounting all of the cutters within the cutting zone on the bit body in the positions so calculated.